.box{
  width: 100%;
  height: 100%;
  overflow: hidden;
  box-sizing: border-box;
}

.pr{
  position: relative;
}
.pa{
  position: absolute;
}
.veralign{
  top: 50%;
  left: 50%;
  transform: translate(-50%,-50%);
}

.hidden{
  display: none;
}

.btn{
  border: none;
  width: 60px;
  min-height: 20px;
  border-radius: 4px;
  box-sizing: border-box;
}
.btn:active{
  opacity: .6;
}